1. Field of the Invention
This invention relates to a support device provided with a vibration isolating structure for reducing vibrations transmitted from a vibration source side to a vibration isolating side.
2. Related Art Statement
In case of transporting or setting precision instruments and the like, or in case of setting power machines and the like generating vibration, there is a vibration isolating support method as a countermeasure for preventing occurrence of such vibrations.
In general, a vibration isolating support device has a structure supporting a cradle together with a rubber isolator, an elastic member such as metal spring, air spring or the like, and if necessary, an attenuator and is interposed between vibration source and vibration isolating means to shut off vibrations.
Such a vibration isolating support device is schematically shown in FIG. 16.
A cradle 03 is supported on a base 01 through a vibration isolating support device 02, and an object 04 is placed on the cradle 03.
Concerning the object 04, there are considered two cases, a first case of which being a vibration isolating body such as a precision instrument and a second case being a vibration generating body such as motor or the like.
The vibration isolating support device 02 comprises plural elastic members and an attenuator and exhibits constant dynamic properties (spring constant K, attenuating factor C) as a whole.
The frequency characteristic of such a vibration transmission ratio (response magnification) in this vibration isolating device is shown in FIG. 17.
As seen from FIG. 17, a large peak of the vibration transmission ratio is shown through resonance at a constant resonance frequency of f.sub.0 =.sqroot.K/M/2n. A vibration isolating area wherein the transmission ratio is not more than 1 is a frequency zone of more than .sqroot.2f.sub.0.
Therefore, the above device has a vibration isolating effect against vibrations having a frequency of more than .sqroot.2f.sub.0 and can isolate vibrations.
Moreover, M is a weight on the vibration isolating device 02 (i.e. total weight of cradle 03 and object 04).
The vibration frequency f of vibration source is resonated in the vicinity of f.sub.0 to inversely amplify vibrations and transmit toward the vibration isolating side, so that it is always required to use the device at the vibration isolating zone of more than .sqroot.2f.sub.0.
That is, the minimum frequency f.sub.min of vibration frequency f of the vibration source side and the resonance frequency f.sub.0 are necessary to satisfy a relationship of .sqroot.2f.sub.0 &lt;f.sub.min.
When the weight M and the vibration frequency f of the vibration source side are previously known, the resonance frequency f.sub.0 satisfying the above relationship is determined, and then the vibration isolating device should be designed so as to obtain such a resonance frequency f.sub.0. However, when the weight M and vibration frequency f are not known, or when they are not constant, it has been difficult to design the optimum device. Therefore, the above vibration isolating device is suitable when the motor or the like is semi-permanently placed, but is not suitable when the loading weight on the cradle is not specified or is varied, or when the vibration frequency f always varies.
For example, in case of a vibration isolating support member for transportation of precision instruments or the like and vibration removing base, the kind, number and the like of the precision instruments loaded on the cradle may vary, so that these devices are designed based on average weight M and vibration frequency f. As a result, the optimum design is not always obtained, and according to circumstances the vibration frequency f may approach to the resonance frequency f.sub.0 to badly exert on the precision instrument.
Therefore, in order to effectively use such a vibration isolating support device, the use condition is restricted, and consequently the general-purpose use is lacking.
Because, the spring constant K in the conventional vibration isolating device itself is fixed and it is usually difficult to properly change the value K adjusted at the designing stage in use.
For this end, there has hitherto been proposed a method wherein plural vibration isolating devices having various spring constants adjusted every loading object are provided and used properly as the conventional countermeasure. However, such a method has drawbacks in cost and efficiency because the plural devices should be used properly.
Furthermore, in order to use the device at a vibration isolating zone of more than .sqroot.2f.sub.0, it is considered that the spring constant K is designed to a considerably low value so as to render the resonance frequency f.sub.0 into a low initial value. In this case, there is no problem when the weight of the loading object is light, but as the weight becomes heavy, the sinking down of the elastomer in the vibration isolating device 02 or deformation under loading becomes considerably large and hence there is caused a problem in the strength and durability of the vibration isolating device 02.
As a result, the weight of the loading object itself is restricted.